Integrated circuits are becoming far more sophisticated while continuing to drop in price. Combinations of both analog and digital functions fabricated on an integrated circuit die, or packaged in a multi-chip package (MCP), are becoming more prevalent and are further increasing the usefulness and reducing the cost of consumer and industrial products. The combination of a microcontroller, and analog and digital circuit functions on an integrated circuit die or in an MCP has also expanded the useful range of applications. Consumer and commercial products, such as, for example, but not limited to, appliances, telecommunications devices, automobiles, security systems, full-house instant hot water heaters, thermostats and the like, are being controlled by integrated circuit microcontrollers. Analog inputs for receiving sensor information and analog outputs for controlling functions are necessary for the application of these microcontrollers. Heretofore separate and discrete analog-to-digital and digital-to-analog interfaces were used to connect the digital microcontroller to the outside analog world.
Analog input devices such as an analog-to-digital converter (ADC) in conjunction with a separate operational amplifier (op-amp) were used to convert a time-varying analog signal into digital representations thereof for coupling to digital inputs and use thereof by the microcontroller. Voltage and current levels were also detected by discrete integrated circuit voltage comparators that changed a digital output state when a certain analog value was present on the input of the comparator.
The operational amplifier (and comparator) is generally a differential input (inverting and non-inverting inputs) analog device, and the circuit of the op-amp has inherent direct current (DC) input offset voltage that causes the output of the op-amp to be nonzero with a zero input voltage between the differential inputs (e.g., inputs connected together). Many applications require an op-amp with a very small input offset voltage. To achieve a small input offset voltage, normally a calibration step in the production of the op-amp is required. This calibration step takes time during manufacturing/testing of the op-amp, and is therefore generally expensive to perform. The calibration is typically performed at one operating point (e.g., temperature, common mode voltage, etc.) such that changes in operating environment, e.g., temperature, voltage, etc., are not compensated for in the manufacturing/testing thereof. Technology has now advance to the point where the analog input and output devices can be fabricated on the same integrated circuit die on which the digital microcontroller, and its support logic and memories are also fabricated. This creates an additional problem in that the equipment used to test the digital microcontroller functions is not capable of performing on-line calibration of analog functions efficiently. Therefore, additional testing equipment and testing steps are required at the time of manufacture. Also the test mode logic and interfacing/multiplexing circuits become more complicated, especially if the number pins (external connections) of integrated circuit package are few in number.
An instrumentation amplifier has sources of error, including offset error, gain error, and circuit parasitics that limit performance thereof. Calibration of the instrumentation amplifier is desirable to reduce these errors, making the instrumentation amplifier suitable for a broader range of applications.